The present invention relates to a shift control system for a toroidal continuously variable transmission (toroidal CVT), which system enables a vehicle to smoothly start running at a lowest-speed gear ratio.
Japanese Patent Provisional Publication No. 5-26317 discloses a cam structure which is arranged to increase an inclination angle of an operation surface of a precession cam in a toroidal CVT so that a feedback gain of a mechanical feedback system is increased in a gear ratio region close to the lowest-speed gear ratio. In case that the feedback gain of the mechanical feedback system is increased in a gear ratio region close to the lowest-speed gear ratio, a shift speed during shifting is suppressed as the actual gear ratio approaches the lowest-speed gear ratio. This arrangement prevents a power roller supporting member of the toroidal CVT from being excessively tilted over a position corresponding to the lowest-speed gear ratio, and therefore this arrangement prevents a collision between the power roller supporting member and a tilt stopper.
Japanese Patent Provisional Publication No. 2001-132827 discloses a control technique for adjusting an actual gear ratio at a desired gear ratio upon taking account of a generation of a torque shift caused after a shifting of a toroidal CVT. More specifically, this control technique is arranged to obtain a command gear ratio by adding the desired gear ratio and a torque shift compensation quantity TSrto, which takes a maximum value when a gear ratio Ratio0 takes a lowest-speed gear ratio xcex11 as shown in FIG. 10 wherein a transmission input torque is constant, and to control a shift control device according to the command gear ratio so as to correspond the actual gear ratio to the desired gear ratio.
Since the torque shift compensation quantity TSrto is adapted to correct an attainable gear ratio toward a high-speed side gear ratio (corresponding to decreasing the attainable gear ratio), it takes a negative value as shown in FIG. 10. When an absolute value of the torque shift compensation quantity is high, it is called and defined that the torque shift compensation quantity is large throughout the specification.
However, if both of the above discussed techniques are employed to improve a shift control system of a toroidal CVT, that is, when the feedback gain of the mechanical feedback system is set a large value to prevent the collision between the power roller supporting member and the tilt stopper as disclosed in the former technique and when the torque shift compensation quantity in a gear ratio region close to the lowest-speed gear ratio xcex11 is set to take a large value as shown in FIG. 10 discussed in the latter technique, a shifting to the lowest-speed gear ratio is incompletely executed due to the excessive suppression by both techniques, without achieving the lowest-speed gear ratio. Accordingly, this simple combination of the former and latter techniques degrades a starting performance for staring the vehicle from a stop state.
It is therefore an object of the present invention to provide an improved shift control system for a toroidal CVT which system employs a shift tactics for solving the above-discussed operational problem.
An aspect of the present invention resides in a shift control system for a toroidal continuously variable transmission (toroidal CVT) of a vehicle. The shift control system comprises a controller which is configured to calculate a command gear ratio of the toroidal CVT by adding a desired gear ratio and a torque shift compensation for compensating a difference between the desired gear ratio and an actual gear ratio which difference is generated by a shifting operation of the toroidal CVT, to set the torque shift compensation quantity so as to increase as the actual gear ratio approaches a low-speed side gear ratio region including a lowest-speed gear ratio and so as to decrease when the actual gear ratio is in the low-speed side gear ratio region.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.